The present invention relates to the use of advanced ceramic materials in the electronic packaging of high power discrete semiconductor devices. Particularly, the invention relates to the use in electronic packaging of aluminum nitride (AlN) substrates which have been processed with a specific pattern of metallization in order to minimize residual stresses present in the substrate due to the manufacturing process for the package, i.e. brazing procedures.
A semiconductor device is incorporated into a microelectronic package to provide functions to the package. Electrical interconnection is provided for the transfer of power and information-bearing signals between the device and the external environment. Mechanical support is provided for durability in further manufacturing, handling and the operating environment. The device is protected from outside contaminants such as moisture, dust, and chemical agents.
High power discrete semiconductor devices such as power transistors, radio frequency and microwave frequency amplifiers, power control switches and the like generate a significant amount of heat in operation, on the order of 10 to 100 watts, and thus require heat dissipation through their carriers, e.g. ceramic substrates, in order to prevent overheating and damage to the device, such as diminished performance or operational failure.
Heat conduction is required as part of the thermal management of the semiconductor device and also for heat experienced in assembly procedures such as die attachment, wire bonding, encapsulation, testing and the like. Heat dissipation is required to cool the devices during use.
To accomplish heat conduction and dissipation, package materials having a high thermal conductivity are employed. These package materials must be carefully selected and incorporated or designed, however, so that thermal cycling does not induce deleterious stresses due to incompatibility of the coefficients of thermal expansion of the various package materials and/or the semiconductor device. Further, the package materials must minimize any adverse effects upon the transmission of power and electrical signals between the semiconductor device and other electronic elements in the system. Characteristics that must be addressed include dielectric constant of the substrate, resistance of the interconnects, and the like.
In applications where high heat dissipation is required, greater than about 5 to 10 watts as is encountered with power discretes, beryllia (BeO) has been used as a substrate material in conjunction with a metal heat sink. The hazardous nature of beryllia makes it undesirable, however. Additionally, the coefficient of thermal expansion of beryllia is about twice that of silicon, making the semiconductor substrate interface subject to mechanical stresses due to thermal or power cycling.
It is therefore desirable to replace the beryllia substrate with an advanced ceramic, such as aluminum nitride, which does not have the hazardous characteristics of Beo and which has a coefficient of thermal expansion (CTE) closer to that of the silicon semiconductor device while meeting other required characteristics.
The replacement of BeO with AlN is complicated, however, due to two factors. While the CTE match between AlN and silicon is closer than that between BeO and silicon, the CTE mismatch between AlN and the metal heat sink is greater than that between BeO and the metal. Also, BeO has a higher thermal conductivity than AlN, making it desirable to decrease the thermal path length between the semiconductor device and the metal heat sink. By reducing the thickness of the AlN ceramic substrate relative to the thickness of the BeO, equivalent thermal performance can be achieved.
The stress between the AlN substrate and the metal heat sink is therefore increased. One way to reduce the stress between the AlN substrate and the metal heat sink is to reduce the area of contact between those package elements. This has limited applicability, however, as too drastic a reduction in area will result in the loss of sufficient heat conduction from the device through the substrate to the heat sink on the substrate's opposite surface.
It is therefore necessary, in order to maintain the structural and operational integrity of the discrete package, to accomodate the stress between the AlN substrate and the metal heat sink by reducing stresses throughout the remainder of the package.
An area in which it is critical to reduce stresses is the area in which the lead frame is bonded to the substrate, as this structure is often the first to fail in a stressed package.
Because of the potential failure of the lead frame to substrate bond, it has been proposed to anchor the lead frame 11 to the substrate 12 by extending the metallization and the solder or braze 13 over the edge of the substrate as depicted in FIG. 1. This results however in the formation of stress region 14 where failure of the bond or the ceramic can occur catastrophically.